Cross-linking of poly



United States atent fiice 3,017,379 Patented Jan. 16, 1952 3 017 379CROSS-LINKING oFPoLYmNYL CHLORIDE) George B. Feild, New Castle, Del.,assignor to Hercules for the reaction. To overcome acidity in thecomposition, carbonates or bicarbonates of alkali metals or alkalineearth metals may advantageously also be added. Calcium carbonate, forexample, enhances the cross-linkazgig g Wllmmgton a corpomuon of ingactivity by preventing acid catalyzed decomposition N0 Drawing. FfiedJune 30 1960, sen 39,817 of the dicumyl peroxide by acid liberated fromthe poly- 3 Claims. c1. 2511-41) (vmyl chlorldel The pentaerythritol andzinc oxide are used in a weight Th s invention relates to thecross-linking of poly(vinyl ratio of :1 to 4:1, preferably about 7:1,and the comchlor de). 10 bination of pentaerythritol and zinc oxide isused in such I t1s known to cross-link poly(vinyl chloride) by radioanamount that the pentaerythritol contained therein is in activeirradlation to improve its properties. A more the range of 6-12 partspentaerythritol per hundred of practical means of cross-linking isdesired. the poly(vinyl chloride). The dicumyl peroxide is used Inaccordance with the present invention it has now in the range of /2 to 6parts per hundred of poly(vinyl been found that poly(vinyl chloride) iscross-linked by chloride), but preferably 2 to 4 parts. heatingpoly(vinyl chloride) with pentaerythritol, in the The following examplesillustrate the manner of carrypresence of zinc oxide and dicumylperoxide or its homoing out the process of this invention for producinguseful 8 compositions and the manner of effecting the cure of the Theprocess of this invention is carried out by mixing composition. Allparts and percentages are by weight. poly(vinyl chloride) with thepentaerythritol, zinc oxide 0 EXAMPLES and dicumyl peroxide as essentialingredients and other ingredients of the composition to form a blend,shaping III the mpl Illustrated here P 1Y( Y Chloridfi) the composition,and curing by heating at a temperature of plastisol grade (66011 wasStirred at room in the range of about 300 F. to 450 F. preferably 350perature into a mixture of the plasticizer, the pentaeryth- F. to 400 F.for a period of time in the range of 0.5 to ritol and zinc oxidemixture, and other dry ingredients. 60 minutes for cross-linking to takeplace. A slow cure at Where a filler was used, it was also added at thisstage. about 350 F. is preferred to a rapid cure at 400 F. in The mixingwas completed with a high powered stirrer. most cases. A two-roll millat 250 F. was useful particularly with a The composition containing theessential components filled composition. The composition was sheeted 'ona may be made without a plasticizer using comminuted two roll mill atabout 250 F., whether initially processed ingredients as an aid in theinitial mixing or using a as a plastisol or milled, and cured betweenheated alumiplasticizing solvent for the poly(vinyl chloride) which isnum plates under a hydraulic pressure of 700-800 p.s.i. to subsequentlyevaporated. However, it is preferable to a thickness of 75 mils.Physical properties were deteruse a plasticizer. Suitable plasticizersare any of the mined on samples cut from the cured sheets. many knownplasticizers for poly(vinyl chloride). Part- Cross-linkingmay be shownby increased zero strength, icularly good plasticizers are esters ofdibasic carboxylic temperature, and by decreased Tinius-Olsen flow dataacids and alcohols of 8 to 18 carbon atoms. Suitable expressed as inchesflow per minute. Only the latter plasticizers include tricresylphosphate, trioctyl phosphate, data are set forth for uniformity in thefollowing exdiisooctyl phthalate, octyl decyl phthalate, diisooctylamples.

Table Example 121 1b 1c 1d 2a 2b 3 4 6a 5b Polyvinyl chloride 100 100100 100 100 100 100 100 100 100 Octyl decyl phthalate 50 50 50 50 50Magnesium oxide Plumb-O-Sil B Dibasic lead stearate Zinc oxide, 1 part;pentaeryth 8 8 8 12 8 8 Dieumyl peroxide 0 2 4 4 0 4 Sterling MT carbonblack 50 50 50 50 100 100 Cure (minutes/ F.) 50/300 00/300 00/300 60/30015/350 15/350 15/350 5/400 15/350 15/350 Tinius-Olsen flow in/mm. 1.131.25 1.00 0. 37 0.78 0.58 0.20 0.08 0.64 0. 00

Example 6a 6b 7 8 9 10 11 12 13a 13b Polyvinyl chloride 100 100 100 100100 100 100 100 100 100 Octyl decyl phthalate 50 50 50 50 50 50 50 50 5050 Calcium carbonate. 3. 5 3. 5 3. 5 0 Zinc oxide, 1 part;pentaerythritol, 7 parts 6 6 6 6 8 8 6 8 24 24 Dicumyl peroxide--. 0 0.5 1. 0 2.0 4 4 2 Cure (minutes/ F.) 15/350 15/350 15/350 15/350 /32555/325 15/350 15/350 5/400 5/400 Tim'us-Olsen flow (in 0. B0 0. 62 O.0.21 0.33 0.25 0. 0. 67 2. 0 1.22

1 Complex of lead ortho silicate and silica gel (51% Pb).

adipate, dioctyl azelate, and dioctyl sebacate. The poly- (vinylchloride) composition may be mixed as a plastisol before heating to cureor it may be completely plasticized to form a homogeneous compositionand subsequently be heated to effect cure.

In the process of this invention, the pentaerythritol appears to enterinto the cross-linking reaction under the influence of the zinc oxidewhich likewise may enter into the cross-linking reaction or act simplyas a promoter for the condensation. The dicumyl peroxide acts as acatalyst Example 1 shows by Tinius-Olsen flow data that pentaerythritoland zinc oxide do not cross-link when dicumyl peroxide is absent(compare 1b with 1d) and that dicumyl peroxide does not causecross-linking when pentaerythritol and zinc oxide are absent (compare 1aand 10 with 1d), while cross-linking does take place (1d) when thesecomponents are present in combination.

Examples 2-5 show filled compositions in which crosslinking is effectedby the combination of pentaerythritol, zinc oxide and dicumyl peroxide.In these examples, 2a

and 5a are blanks lacking the dicumyl peroxide catalyst so as to providea basis for comparing Tinius-Olsen flow.

Examples 6a and b, 7 and 8 show the effect of dicumyl peroxide level oncross-linking as indicated by Tinius- Olsen flow data. The change indegree of cross-linking is greatest between 1.0 and 2.0 parts perhundred of resin.

Examples 9 and 10 exemplify plastisol formulations and milledformulations, respectively, using in each case a plastisol gradepoly(vinyl chloride). The results indicate aboutequivalent'cross-linking in each case.

Examples 11, 12, 13a and 13b show the effect of an excess ofpentaerythritol in combination with zinc oxide. While 6 partspentaerythritol-Zinc oxide mixture gives a product having a Tinius-Olsenflow of 0.21 inch per minute, 8 parts pentaerythritol-zinc oxide mixturegives a product having a Tinius-Olsen flow of 0.67 inch per minuteindicating that the larger amount of pentaerythritol gave lesscross-linking. Similarly, in Example 13a, 24 parts pentaerythritol-zincoxide mixture gave a product of-suchhigh Tinius-Olsen flow as toindicate that the pentaerythritol was present in such an excess as toprovide one pentaerythritol for each bond instead of one pentaerythritolfor every two bonds as would be the case in a cross-linking reaction. Acomparison of the Tinius- Olsen flow for the products of 13a and 13b inwhich no catalyst was used further indicates that there was a reactionproduct in 13a even though not a cross-linking reaction.

While the above examples used a plastisol grade of poly(vinyl chloride)so as to show comparisons more exactly, the poly(vinyl chloride) may bea plastic grade if the compositions are to be produced by mixing andmilling operations. Cross-linking by the process of this invention isequally adaptable to either type of poly(vinyl chloride).

The cross-linking process of this invention greatly improves the heatresistance of the poly(vinyl chloride), and the cross-linked poly(vinylchloride) compositions are advantageously used in-protective coatings ofmany sorts, particularly in the coating of wires for use in electricalwork.

When bis(a,a-dimethyl-p-methyl) peroxide and his-(a,a-dimethyl-p-isopropyl) peroxide were substituted for dicumylperoxide in the above examples, improvements of similar magnitude wereobtained.

The homologs of dicumyl peroxide which are included in thescope of thepresent invention are all peroxides of the formula R2 R R1-0-o-0 -R,

3 5 in which R and R are phenyl and lower alkyl substituted phenyl and RR R and R are hydrogen or lower alkyl, said lower alkyl containing 1 to4 carbon atoms. The preferred peroxides are those in which R R R and Rare lower alkyl, and the preferred example of this group is dicumylperoxide. Specific preferred peroxides of this invention besides dicumylperoxide are: biS(oc,adimethyl-p-methylbenzyl) peroxide andbis(a,a-dimethylp-isopropylbenzyl) peroxide which are higher homologs ofdicumyl peroxide.

What I claim and desire to protect by Letters Patent is:

1. The method of curing poly(vinyl chloride) which comprises heatingpoly(vinyl chloride) in contact with /2 to 6 percent of a peroxide ofthe formula in which R and R are selected from the group consisting ofphenyl and lower alkyl substituted phenyl, and R R R and R are selectedfrom the group consisting of hydrogen and lower alkyl radicals, saidlower alkyl containing 1 to 4 carbon atoms, and 6 to 12 percent ofpentaerythritol, and zinc oxide in the pentae-rythritol-zinc oxideweight ratio of 10:1 to 4:1 at a temperature in the range of about 300F. to about 450 F., said percents being expressed as parts per hundredpoly(vinyl chloride) by weight.

2. The method of curing poly(vinyl chloride) which comprises heatingpoly(vinyl chloride) in contact with /2 to 6 percent dicumyl peroxide, 6to 12 percent of pentaerythritol, and zinc oxide in thepentaerythritolzinc oxide weight ratio of 10:1 to 4:1 at a temperaturein the range of about 300 F. to about 450 F., said percents beingexpressed as parts per hundred poly(vinyl chloride) by weight.

3. The method of curing poly(vinyl chloride) which comprises heatingpoly(vinyl chloride) in contact with 2 to 4 percent dicumyl peroxide, 6to 12 percent of pentaerythritol, and zinc oxide in thepentaerythritol-zinc oxide weight ratio of 7:1 at a temperature in therange of about 350 F. to about 400 F., said percents being expressed asparts per hundred poly(vinyl chloride) by weight.

Zimmerman et aL: Compounding of'Polyvinyl Chloride, Rubber Age, volume68, No. 3, pages 311-318, December 1950.

1. THE METHOD OF CURING POLY(VINYL CHLORIDE) WHICH COMPRISES HEATINGPOLY(VINYL CHLORIDE) IN CONTACT WITH 1/2 TO 6 PERCENT OF A PEROXIDE OFTHE FORMULA